State Key Laboratory of Grassland Agro-ecosystems, Key Laboratory of Grassland Livestock Industry Innovation, Ministry of Agriculture and Rural Affairs, Engineering Research Center of Grassland Industry, Ministry of Education, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou, 730020, China.
Flathead Lake Biological Station, University of Montana, Polson, Montana, 59860, USA.
Ecology. 2019 Aug;100(8):e02755. doi: 10.1002/ecy.2755. Epub 2019 May 30.
Nitrogen (N) and phosphorus (P) are key growth-limiting nutrients for organisms and their absolute and relative supplies regulate the structure and function of ecosystems. Landcover changes lead to modifications of terrestrial biogeochemistry, consequently influencing aquatic nutrient conditions. This study sought to evaluate the potential impacts of grassland degradation on nutrient availability and nutrient limitation in the Qinghai Lake (China) and its inflow streams. We sampled nutrient concentrations and tested stream nutrient limitation by conducting nutrient diffusing substrata (NDS) bioassays in streams flowing through subbasins with different grassland status. To test nutrient limitation and the responses of lake phytoplankton to stream inflows, bioassays were conducted by adding different nutrients (N, P, and joint NP) as well as water from different streams to lake water with phytoplankton, respectively. In general, N concentrations as well as N:P ratios decreased while P concentrations increased with decreased normalized difference vegetation index (NDVI, an index of vegetation status), especially in September, suggesting that grassland degradation (low NDVI) has the potential to differentially decrease N availability and increase P availability in streams. Consistent with this, relative responses (RR) of stream periphyton to P and combined NP enrichments in the NDS bioassays decreased with stream P concentrations while increased with stream water N:P ratios. Lake phytoplankton responded strongly to P and combined NP addition indicating strong P-limitation of lake phytoplankton. RR of lake phytoplankton to stream water decreased with nitrate concentration and N:P ratios in stream water and increased with the concentrations of ammonium, total phosphorus, and soluble reactive phosphorus, indicating that stream water with higher P but lower N and N:P from degraded subcatchments is associated with increased impact on P-limited Lake phytoplankton. Overall, this study suggests that grassland degradation has the potential to differentially influence the nutrients delivered to streams with substantial increases in P but decreases in N and N:P, alleviating P limitation of stream periphyton and, ultimately, stimulating P-limited phytoplankton growth in the lake.
氮（N）和磷（P）是生物生长的关键限制养分，它们的绝对和相对供应调节着生态系统的结构和功能。土地覆盖变化导致陆地生物地球化学的变化，从而影响水生养分条件。本研究旨在评估草地退化对青海湖（中国）及其入流溪流中养分可利用性和养分限制的潜在影响。我们采样了养分浓度，并通过在具有不同草地状况的子流域中流动的溪流中进行养分扩散基质（NDS）生物测定来测试溪流养分限制。为了测试养分限制以及湖泊浮游植物对溪流流入的响应，我们分别通过向含有浮游植物的湖泊水中添加不同的养分（N、P 和 NP）以及来自不同溪流的水来进行生物测定。一般来说，随着归一化差异植被指数（NDVI，植被状况的指标）的降低，N 浓度以及 N:P 比值降低，而 P 浓度升高，尤其是在 9 月，这表明草地退化（低 NDVI）有可能降低溪流中 N 的可利用性并增加 P 的可利用性。与此一致的是，在 NDS 生物测定中，溪流周丛生物对 P 和 NP 联合富化的相对响应（RR）随着溪流 P 浓度的降低而降低，随着溪流水 N:P 比值的增加而增加。湖泊浮游植物对 P 和 NP 联合添加的响应强烈，表明湖泊浮游植物受到强烈的 P 限制。湖泊浮游植物对溪流水的 RR 随着溪流水中硝酸盐浓度和 N:P 比值的降低而降低，随着铵、总磷和可溶反应性磷浓度的增加而增加，这表明来自退化子流域的具有较高 P 但较低 N 和 N:P 的溪流水与增加对 P 限制的湖泊浮游植物的影响有关。总的来说，本研究表明，草地退化有可能对输送到溪流的养分产生不同的影响，溪流中 P 的增加而 N 和 N:P 的减少，减轻了溪流周丛生物的 P 限制，并最终刺激了湖泊中 P 限制的浮游植物生长。
